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of Mr. Lubbock* are against the supposition that the ephippia of Daphnia are gemmæ. He has traced the formation of the ephippial ova in the ovary as true ova ; but admits at the same time that they are possibly agamic. The external resemblance of these ephippia with the statoblasts of the Polyzoa is singularly striking; but, with the very different origin of the two sets of bodies, we must take care not to attribute a greater share of significance to this resemblance than it really deserves.t
The reproductive phenomena of the fresh-water Polyzoa may be thus classed under three distinct heads:
By true ova.
term of their destined development.
developmental activity remains for a
In Cristatella and Lophopus I have frequently witnessed the multiplication of a colony by a process of self-division. In Cristatella this commences by a constriction which takes place generally towards the middle of the colony, and which gradually deepens till at last it divides the entire mass into two separate portions, which move off in opposite directions. In Lophopus the process is very similar ; large specimens of this polyzoon have the endocyst constricted at intervals so as to give to the colony the appearance of a variously lobed body enveloped by the gelatinoid ectocyst. It is at the point of these constrictions that the selfdivision takes place, separating the entire colony into two or more smaller ones.
It may, perhaps, be thought that I ought to have enumerated this multiplication of colonies by a process of self-division as a fourth form of reproduction ; a little consideration, however, will show that this is nothing more than a reproduction by buds, with the separation of the buds in masses. It is analogous to the gemmiparous reproduction of Hydra, and must not be confounded with the true fissiparous reproduction of the lower forms of simple animals. In the Polyzoa the colony thus extends itself by the production of gemmæ, which after development remain permanently adherent; it establishes new colonies by ova and statoblasts, and by ordinary gemmæ which ultimately become detached.
If we attempt to correlate the individual phenomena now described in connection with the reproduction of the Polyzoa, we cannot but be struck with some remarkable analogies which would seem to bring the whole process of generation and gemmation in these animals within the domain of the so-called “Law of alternation of generations.” We have, first, as the immediate result of the development of the ovum, a ciliated sac-like embryo, resembling in form and habit an infusorial animalcule: it is a non-sexual zooid. From this is produced subsequently, by a process of gemmation, another form of zooid, namely, the polypide, with a
* An account of the two methods of reproduction in Daphnia, and of the structure of the ephippium.” By John Lubbock, Esq. Abstract in 'Proc. Roy. Soc.,' Jan. 29, 1857, vol. viii.
+ In the mode of development of the statoblasts from the funiculus of the polypide, we are involuntarily reminded of the development of the chains of salpa-buds from the stolon of the solitary salpæ.
much more highly differentiated structure, in which the organs of digestion especially hold a dominant position, and which we may regard as sexual or non-sexual, according to the view we take of the relation between it and the testis, as will presently be seen.
Now, if the formation of the ovary be attended to, it will be seen that this body is developed at a late period from the walls of the original sac-like embryo, which have undergone slight changes, and have become the endocyst of the more mature Polyzoon ; and it will be at once perceived that this development of the ovary takes place in a way which may obviously be compared with the formation of a bud; that at least in Alcyonella it occupies exactly the position in certain cells that the buds destined to become polypides do in others, and that at an early stage of polypide and ovary it is scarcely possible to distinguish one from the other; so that the idea is immediately suggested, that the body here called ovary is itself a distinct zooid, in which the whole organization becomes so completely subordinate to the reproductive function as to be entirely masked, and apparently replaced by the generative organs. This would then constitute a third zooid, which would therefore be a sexual zooid; it is, however, unisexual (female).
In the next place we find that upon the funiculus (in Alcyonella), which probably belongs rather to the “polypide than to the endocyst, there is developed the mass here described as testis. Now, if we view this mass as a mere organ of the polypide, we must then regard the latter as the second sexual or male zooid; but the testis may perhaps be more correctly considered, like the ovary, as a distinct sexual bud, having the generative system so enormously predominant as to overrule and replace all the rest of the organization ;* this bud, like the ovary-bud, being also unisexual, but with a male function. In confirmation of this view, it is to be remembered that the funiculus has the power of giving origin to a very remarkable form of undoubted bud, the statoblast, which, until ulterior development is excited in it, has no nearer resemblance to an ordinary polypide-bud than the testicular mass has; and to this statoblast-so far at least as position is concerned—the male bud or testis in Alcyonella would therefore be related just as the female bud or ovary is related to an ordinary polypide-bud. In Paludicella the testis, though in immediate connection with the funiculus, is developed apparently from the endocyst.
If the above be the correct view, the complete comprehension of the Polyzoon will involve the conception of a ciliated sac-like embryo as a starting point, and a series of buds, of which the last term will consist of a pair of sexual buds, the others being non-sexual ; from the sexual buds a true embryo like the first is again produced, which affords the point of departure for another similar cycle.
* Analogous instances of the dominant development of the generative system, so as to suppress more or less completely the development of all the other organs, occur in other members of the animal kingdom; as examples, may be mentioned the reproductive capsules (true buds) in certain Polypi, and the male of some of the Rotiferæ.
HomologIES OF THE POLYZOA.
Before commencing the zoographical portion of the present memoir, there still remains to be considered a subject of great interest to the philosophical zoologist, namely, the exact position and affinities of the Polyzoa in the animal kingdom, a question which admits of much valuable elucidation from the study of the freshwater genera.
We have seen in the historical sketch already given of the successive steps which resulted in the separation of the Polyzoa from the Polypes, that the molluscan relations of the Polyzoa began at last to be recognised in an obvious resemblance between their organization and that of the Ascideæ. Guided by this relation, an important step was finally taken by M. MilneEdwards in distinguishing two primary series in the mollusca, characterised mainly by the relative grade of perfection of the nervous and circulatory systems. One of these, the Molluscoida, included the Polyzoa and Tunicata ; the other, the Mollusca proper, embraced all the other members of the molluscan sub-kingdom. The affinities of the Tunicata and Polyzoa being thus fully recognised, it is of importance to know in what these affinities really consist; in other words, to determine the homologous organs in the two groups.
I had elsewhere* attempted to show what I believed to be the correct view of the relation between the Tunicata and the Polyzoa. Additional opportunities of investigation have suggested a few modifications, but after a careful comparison of all that has been urged on this subject, I am still of the opinion that in its leading points' view then taken was the true
The great respiratory sac of the Tunicata is the most striking feature in the structure of this group, and I shall therefore first endeavour to point out where the respiratory sac of the Tunicata is represented in the Polyzoa.
Now, the opinions entertained on this subject may be classed under two distinct heads. Under the one head it is maintained that the respiratory sac of the Ascidian has its representative in the pharynx of the Polyzoon, and that the rudimentary tentacula at the orifice of this sac are homologous with the tentacula of the Polyzoon. Under the other head it is asserted that the respiratory sac of the Ascidian is homologous with the tentacular crown of the Polyzoa. I have carefully examined all that has been urged in favour of each of these views, and I believe that the evidence preponderates on the side of considering the respiratory sac of the Ascidian as truly represented by the tentacular crown of the polyzoon, though the exact nature of the homology has not been correctly stated. The reader will call to mind that the respiratory chamber of an Ascidian, consists essentially of a membranous sac, having the inner surface of its walls covered by two sets of tubular bars, one running longitudinally, or from behind forwards, the other transversely, or from the neural to the hæmal side, and thus crossing each other at right angles, while the membranous wall of the sac, in every one of the
* On the Homology of the Organs of the Tunicata and the Polyzoa. “Transactions of the Royal Irish Academy, vol. xxii, 1852.
quadrangular meshes thus formed, is perforated by an aperture, (the stigmates branchiaux of Milne-Edwards),* surrounded by a fringe of vibratile cilia.
It must, moreover, be borne in mind, that the transverse bars open into two great longitudinal canals, the “branchial” and “thoracic sinuses” of Milne-Edwards, one placed upon the neural, and the other upon the hæmal side of the sac, that they constitute, in fact, a series of communicating channels, passing transversely between these two sinuses, receiving the blood from one and pouring it into the other.
In advocating the homology of the respiratory sac of the Ascidian with the tentacular crown of the Polyzoon, some zoologists, among whom must be especially mentioned M. Van Benedenf, maintain that the longitudinal bars of the sac correspond to the tentacula of the Polyzoon, while the transverse bars in the sac of the Ascidian must be considered as becoming extinct in the tentacular crown of the Polyzoon. Now this view is certainly untenable. I have in another place* endeavoured to show that while the respiratory sac of the Ascidian has undoubtedly its homologue in the tentacular crown of the Polyzoon, it is the transverse and not the longitudinal bars of this sac that are represented by the tentacula of the Polyzoon, Fig. 6.
Fig. 6. Plan of an Ascidian Tunicate
Fig. 7. Plan of an Ascidian Tunicatelongitudinal section.
transverse section. a. External tunic. 6. Middle tunic. c. Internal tunic. d d d. General sinus system. e. Respiratory
orifice. f. Cloacal orifice. g. Transverse respiratory bars. h. Longitudinal respiratory bars.
t. Tentacula at entrance of respiratory chamber. u. Ganglion. v. Heart. * See Milne-Edwards's beautiful memoir, “Sur les Ascidies Composées." † Van Beneden sur les Ascidies simples. "Mém. de l'Acad. Roy. de Belgique,' tome xx, 1847. I Trans. Roy. Irish Acad., Jan, 1852.
and this is a very important distinction, which from not having been recognised, rendered previous attempts at comparison between the respiratory sac of the Ascidian and the tentacular crown of the Polyzoon indefensible.
Without a knowledge of the Hippocrepian Polyzoa, it would, perhaps, have been impossible to arrive at anything like a satisfactory conclusion on this point; the peculiarities of these Polyzoa, however, afford a key to the clearing up of this difficult subject, and we shall best perceive the relations in question by comparing an Ascidian Tunicate with a Hippocrepian Polyzoon, a Clavelina, for example, with a Plumatella, a comparison which the accompanying diagrams (figs. 6–9) will render easy. * Now it does not need much assistance from the imagination to see in the great branchial sinus of Clavelina, a representative of the lophophore of Plumatella, while the transverse bars which pass off at either side from this sinus, and are richly ciliated, will correspond to the ciliated tentacula of the Polyzoon ; the delicate membrane which constitutes the proper walls of the respiratory sac, to the interior of which the respiratory bars of the Ascidian are adherent, and which is pierced in the intervals of these bars by the “branchial stigmata,” will have its homologue in
a. Ectocyst. b. Endocyst. c. Tantacula sbeath.
ddd. Perigastric space.
f + e. Orifice of cell. g. Tentacula. i. Lophophore. 1. Calyx. m. Epistome. n. Mouth. o. Esophagus. p. Stomach. 9. Intestine. r. Anus.
8. Cavity of tantacular sheath. u. Ganglion. w. Retractor muscle.
* In figs. 6-11, the same letters are used throughout to indicate homologous parts.